Magnetic bubble memories are well known in the art. One mode of propagating bubbles is called a "field access" mode employing a rotating magnetic field as disclosed in A. H. Bobeck, U.S. Pat. No. 3,534,347, issued Oct. 13, 1970.
A familiar organization for a magnetic bubble field access memory employs a major/minor configuration as disclosed in P. I. Bonyhard, U.S. Pat. No. 3,618,054, issued Nov. 2, 1971. The basic major/minor configuration utilizes a major accessing loop, and minor loops which serve as permanent stores. A variety of other major/minor configurations are also well known in the art. Several are described in, for example, "Systems Analysis for Spaceborne/Airborne Magnetic Bubble Mass Memory", Rockwell International, Report No. AFAL-TR-74-270, contract or grant No. F33615-73-C-1103.
In one particularly attractive major/minor configuration a bubble generator and a bubble detector constitute part of a major loop and the minor loops are accessed by the major loop through transfer in/out gates. A problem with this type of configuration is that data cannot be accessed until the next preceding data are transferred back into the minor loops. During power failure, data in the major loop may be lost because the address of the data cannot be stored in this configuration or in the controller without considerable additional circuitry.
In another major/minor configuration similar to that disclosed in the above-mentioned Bonyhard patent, there are two major paths instead of a major loop to which the data is transferred. The first major path contains a generator which generates bubbles which are then swapped with data previously stored in the minor loops for permanent storage there. The second major path contains a detector which detects bubbles replicated into the path from the minor loops.
This last-mentioned configuration saves data from being lost during a power failure because the data are replicated rather than transferred into a major path for detection and thus never leave safe positions in the minor loops. Unfortunately, because of the number of replicate ports or gates used in the configuration, a large voltage is required to drive the replicators. Moreover, should a power failure occur, the resynchronization of data requires additional housekeeping circuitry.
A failsafe, non-volatile bubble memory can be designed with a housekeeping loop in addition to the major and minor loops of a bubble configuration. One such housekeeping loop is disclosed in J. L. Smith, U.S. Pat. No. 3,508,225, issued Apr. 21, 1970. In that patent, an additional bubble propagation channel is operative as a counter having a known number of stages between a bubble generator and a detector. Because bubbles propagate one position each time a field rotates one cycle, a magnetic bubble is advanced stage by stage in the counter in synchronism with the propagation or bubbles introduced into the major loop from the minor loops. A loss of power merely stops a bubble in the counter. When power is next supplied, that bubble advances to the detector supplying, then, the address for the word in the major loop. But this arrangement requires additional driver and detector implementations. The problem to which this invention is directed is, thus, to obtain a relatively fast bubble memory which is secure from power failure problems without requiring block replication or additional implementation.